1. Field of the Invention
The present invention relates to a method of, and apparatus for, examining an object by means of penetrating radiation, such as X-rays or gamma-rays.
2. Description of the Prior Art
Considerable effort has been expended in recent years to develop a method and apparatus for accurately examining the detailed structure of the interior of a body by penetrating radiation which utilize principles of tomographic reconstruction. These efforts have been directed toward solving the problem of obscured details found in conventional X-ray or gamma-ray radiographic imaging. Such obscuring of details was caused by the superimposition of the other structural details of the body through which the radiation passed prior to detection.
The more recent developments are based upon the theoretical principles of computer-assisted tomography which have been recently summarized in a paper entitled "Principles of Computer Assisted Tomography (CAT) in Radiographic and Radioisotopic Imaging" by Rodney A. Brooks and Giovanni Di Chiro, published in Vol. 21, No. 5 at pp. 689-732 of PHYS. MED. BIOL. (1976) Specific methods and apparatus developed to employ these principles for medical applications are exemplified by U.S. Pat. Nos. 3,778,614; 3,866,047; 3,944,833; and 3,924,131. These patents disclose a method of exposing a planar slice of a body to penetrating radiation along a plurality of paths at different mean angular positions and detecting the radiation emerging from the body to estimate the radiation absorbed by the body along each path. The mean angular positions of each of the plurality of paths are so arranged so that such paths intersect every element of a two-dimensional array of elements (matrix) used to delineate the planar slice of the body.
The data representative of the radiation absorbed along each of the paths is then used to estimate the absorption coefficients of each element of the matrix by employing either iterative reconstruction techniques, as exemplified by U.S. Pat. No. 3,778,614; analytic reconstruction techniques (convolution function), as exemplified by U.S. Pat. No. 3,924,129; and combination analytic/iterative techniques, as exemplified by U.S. Pat. No. 3,924,129.
Various apparatus have also been developed to carry out the computer-assisted tomographic methods described above. In U.S. Pat. No. 3,924,131 a radiation source and a detector are mounted on a frame facing each other across an aperture in which the body is positioned so that the source and detector are moved together relative to the stationary body between different lateral positions so that the collimated radiation beam is scanned laterally to form a plurality of paths and then the frame rotated to other angular positions where additional lateral scanning is effected. In U.S. Pat. Nos. 3,866,047 and 3,944,833 a source transmitting a collimated fan beam of radiation toward the object and a bank of detectors positioned opposite the source are mounted on a turntable for orbiting the source and bank of detectors relative to the body. Other scanners have been developed which utilize a stationary circular array of detectors and the rotation of a single source along a circular path just within the circular array of detectors. All of these apparatus, however, involve movement of the source or detectors, or both, relative to the body being examined during a scan of a slice of the body. Such movement also limits the speeds at which the scanning can be accomplished. For example, movement makes it difficult to exactly determine the coordinate position of the ray paths, determine the exact position of the moving source or detector due to aberrations in the position of the source or detectors caused by the inherent variable movement of heavy mechanical devices when they are in motion, and high voltage connection problems caused by rotation of the source and/or detectors. Each of these problems reduce the accuracy of the data obtained for use in determining the absorption coefficients of the matrix elements.
Previous examining apparatus has also attempted to represent the radiation intensity detected by generating output signals representative of the total electrical charge generated by the detection system during an exposure, as exemplified by U.S. Pat. Nos. 3,778,614; 3,124,131; 3,866,047, and 3,956,633. Use of such current integration techniques to obtain output signals require high dosage levels in order for the intensity level emerging from the body to be detectable over the system's noise. The current integrating apparatus also induces measurement errors in the output signal. Moreover, the accuracy of the output signal is further degraded since the current integrated signal fails to account for statistical variations in the energy content of the photons detected. As a result such measurements cannot be used in applications to determine the exact number of photons detected.
Previous examining apparatus employing multiple detectors has also attempted to segregate the paths of radiation passing through the body to the detectors by using relatively bulky mechanical collimators, as exemplified by U.S. Pat. No. 3,866,047. Such collimators prohibit the placement of detectors in high-density detector arrays, thereby requiring a longer exposure time or an increase in the number of scans in order to obtain sufficient information for an adequate reconstruction.
Other apparatus and methods employed in the examination of a body using reconstructive tomography are illustrated in U.S. Pat. Nos. 3,778,614; 3,866,047; 3,944,833; 3,867,634; 3,919,552; 3,924,131; 3,881,110; 3,965,357; 3,936,636; 3,924,129; 3,932,757; 3,952,201; 3,973,128; 3,946,234; and 3,956,633.
Finally, recent scientific experiments have been conducted to evaluate the feasibility of using plastic scintillators for low energy, high rate photon detection, as shown by an article entitled "Comparative Studies on Plastic Scintillators--Applications to Low Energy High Rate Photon Detection" by L. A. Eriksson, C. M. Tsai, Z. H. Cho, and C. R. Hurlbut, published at pp. 373-376 in Vol. 122 of NUCLEAR INSTRUMENTS AND METHODS (1974). The experiments discussed in the above-referenced article, however, were conducted with only monoenergetic radiation. Furthermore, so far as is known, there has been no previous use of such high-speed plastic scintillators in apparatus for tomographic examination of an object by means of penetrating radiation.